1. Field of the Invention
The present invention relates to a switching power supply which is highly efficient in operation.
2. Description of the Related Art
FIGS. 1 and 2 of the accompanying drawings show conventional switching power supplies. The conventional switching power supply shown in FIG. 1 is of the general forward type. For making a highly efficient switching power supply, particularly a switching power supply with a low-voltage (e.g., 5 V or 3.3 V), high-current output capability, using the circuit shown in FIG. 1, since rectifying diodes D.sub.101, D.sub.102 connected to the secondary winding N.sub.12 of a transformer T.sub.101 cause a large power loss, it is often customary to employ synchronous rectifying MOSFETs in place of the rectifying diodes D.sub.101, D.sub.102. It is of importance to consider how these synchronous rectifying MOSFETs (also referred to as synchronous rectifying transistors) are to be driven to achieve a highly efficient switching power supply.
The conventional switching power supply shown in FIG. 2 employs synchronous rectifying transistors Q.sub.102, Q.sub.103 instead of the rectifying diodes.
The conventional switching power supply shown in FIG. 1 which employs the rectifying diodes suffers the same problems as those of the conventional switching power supply shown in FIG. 2 which employs the synchronous rectifying transistors, except that the synchronous rectifying transistors are driven in the conventional switching power supply shown in FIG. 2. Therefore, the conventional switching power supply shown in FIG. 2 will be described below.
FIG. 3 of the accompanying drawings shows the waveforms of voltages and currents in various parts of the conventional switching power supply shown in FIG. 2. FIG. 4 of the accompanying drawings shows an output voltage of the conventional switching power supply shown in FIG. 2 with respect to a duty cycle (the ratio of an on-time to an operating period of a switching element Q.sub.101) thereof.
In FIG. 3, T.sub.1 represents an operating period of the switching element Q.sub.101, T.sub.ON1 represents an on-time thereof, and T.sub.off1, T.sub.off2 represent an off-time thereof.
V.sub.gs (Q.sub.101), I.sub.d (Q.sub.101), and V.sub.ds (Q.sub.101) represent agate voltage, a drain current, and a drain-to-source voltage, respectively, of the switching element Q.sub.101, and V(N.sub.11) represents a voltage across the primary winding N.sub.11 of the transformer T.sub.101.
Of the voltage V(N.sub.11) across the primary winding of the transformer T.sub.101, a voltage V(.sub.1) in the off-time T.sub.off1 of the switching element Q.sub.101 is generated to reset the transformer T.sub.101, after it has been excited in the on-time T.sub.ON1 of the switching element Q.sub.101. The voltage V(h.sub.1) is generated such that an integral of the voltage with respect to time in the on-time will be equal to an integral of the voltage with respect to time in the off-time.
The waveform of the voltage V(N.sub.11) in the off-time T.sub.off1 is determined depending on the magnetizing inductance and the capacitance between output terminals of the switching element Q.sub.101. When the voltage of an input power supply V.sub.in and an output current supplied to a load vary, the duty cycle of the switching element Q.sub.101 varies to keep the output voltage constant, and the voltage V(h.sub.1) and the off-times T.sub.off1, T.sub.off2 also vary.
In order to reset the transformer T.sub.101 in the off-time T.sub.off1 by exactly the same quantity as it has been excited in the on-time T.sub.ON1 under any input and output conditions, it is necessary to sufficiently provide the off-time T.sub.off2 in which no voltage is induced across the primary winding N.sub.11, after the resetting of the transformer T.sub.101. As described later on, the need to increase the off-time T.sub.off2 poses a serious problem.
In FIG. 3, V.sub.ds (Q.sub.102) represents a drain-to-source voltage of the synchronous rectifying transistor Q.sub.102 , and V.sub.ds (Q.sub.103) represents a drain-to-source voltage of the synchronous rectifying transistor Q.sub.103. These voltages are voltages converted from the voltage V(N.sub.11), in the respective off-and on-times T.sub.off1, T.sub.ON1, across the primary winding N.sub.11 of the transformer T.sub.101 with the turns ratio of the primary and secondary windings N.sub.11, N.sub.12 of the transformer T.sub.101.
One major problem encountered in making a highly efficient switching power supply using the circuit shown in FIG. 2 is that since the drain-to-source voltage V.sub.ds (Q.sub.102) of the synchronous rectifying transistor Q.sub.102 is large as shown in FIG. 3, the synchronous rectifying transistor Q.sub.102 has a large on-state resistance and causes a large power loss, resulting in a reduction in the switching power supply efficiency.
Specifically, though the synchronous rectifying transistor Q.sub.102 should have a smaller on-state resistance for higher switching power supply efficiency, MOSFETs have such a general tendency that their on-state resistance is higher as the drain-source breakdown voltage is higher.
The voltage V(N.sub.11) in the off-time T.sub.off1 across the primary winding N.sub.11 of the transformer T.sub.101 is of a sine wave because it resonates with the magnetizing inductance and the capacitance between the drain and source of the switching element Q.sub.101, and hence has a large maximum level. Furthermore, since the voltage V(N.sub.11) varies greatly depending on the input and output conditions, the synchronous rectifying transistor Q.sub.102 is required to have a large dielectric strength between the drain and source thereof and hence a large on-state resistance.
The above problem holds true for the conventional switching power supply shown in FIG. 1 where the rectifying diode D.sub.101, is used in place of the synchronous rectifying transistor Q.sub.102.
The problem of the dielectric strength of the synchronous rectifying transistor Q.sub.102 is also the problem of the dielectric strength of the switching element Q.sub.101. One conventional way of limiting the dielectric strength to a certain voltage is to use a clamping circuit, which comprises a diode, a capacitor, and a resistor, between the terminals of the primary winding N.sub.11 of the transformer T.sub.101. Though the clamping circuit is capable of clamping the voltage to a certain level, however, the efficiency is lowered because the magnetization energy of the transformer T.sub.101 is consumed by the resistance of the clamping circuit. According to another conventional way of clamping the voltage, a tertiary winding is added to the transformer T.sub.101, and connected to the input power supply V.sub.in via a diode, thus providing a clamping circuit.
With the latter conventional clamping circuit, most of the excitation energy of the transformer T.sub.101 flows to the input power supply V.sub.in. When this current flows through the diode of the clamping circuit, the diode develops a voltage drop which causes a power consumption resulting in a reduction in the efficiency. In addition, the transformer T.sub.101 is large in size and complex in structure because of the added tertiary winding, and the tertiary winding suffers an increased conduction loss.
Another drawback which results from making a highly efficient switching power supply using the circuit shown in FIG. 2 is that because of the off-time T.sub.off2 shown in FIG. 3, the synchronous rectifying transistor Q.sub.103 cannot be energized for the entire period in which the switching element Q.sub.101 is turned off, resulting in a reduction in the switching power supply efficiency. This drawback is inherent in using synchronous rectifying transistors, and is the most serious in the manufacture of highly efficient switching power supplies. In the on-time T.sub.ON1 of the switching element Q.sub.101, the synchronous rectifying transistor Q.sub.103 is turned off and the synchronous rectifying transistor Q.sub.102 is turned on, and a current through a choke coil L.sub.10 flows through the synchronous rectifying transistor Q.sub.102. In the off-time T.sub.off1 of the switching element Q.sub.101, the synchronous rectifying transistor Q.sub.102 is turned off and the synchronous rectifying transistor Q.sub.103 is turned on, and a current through a choke coil L.sub.10 flows through the synchronous rectifying transistor Q.sub.103.
The gate terminals of the synchronous rectifying transistors Q.sub.102, Q.sub.103 are energized by the voltage across the secondary winding N.sub.12 of the transformer T.sub.101. When one of the synchronous rectifying transistors Q.sub.102, Q.sub.103 is turned on, the gate voltage thereof comes from the drain-to-source voltage of the other synchronous rectifying transistor which is turned off.
The switching element Q.sub.101 has the off-times T.sub.off1, T.sub.off2. As can be seen from FIG. 3, since the drain-to-source voltage V.sub.ds (Q.sub.102) of the synchronous rectifying transistor Q.sub.102 has a certain level in the off-time T.sub.off1, the synchronous rectifying transistor Q.sub.103 can be energized in the off-time T.sub.off1. However, in the off-time T.sub.off2, the synchronous rectifying transistor Q103 cannot be energized because the drain-to-source voltage V.sub.ds (Q.sub.102) is nil. Therefore, the synchronous rectifying transistor Q.sub.103 is turned off in the off-time T.sub.off2. During this time, a current through the choke coil L.sub.10 flows through a body diode of the synchronous rectifying transistor Q.sub.103, i.e., a parasitic diode inserted from the source terminal to the drain terminal thereof because of the MOSFET structure. Inasmuch as a voltage drop across the body diode is much greater than a voltage drop caused when the synchronous rectifying transistor Q.sub.103 is turned on, the power loss in the off-time T.sub.off2 is increased, reducing the switching power supply efficiency.
Still another shortcoming is that the choke coil of the output filter is large because an output ripple voltage is high. Stated otherwise, if a chock coil of a certain size is used to keep the output ripple voltage to a prescribed level, then the iron loss and copper loss of the chock coil are increased, indirectly lowering the efficiency of the switching power supply.
In FIG. 3, the drain-to-source voltage V.sub.ds (Q.sub.103) of the synchronous rectifying transistor Q.sub.103 is the same as a voltage V(P) at a point P shown in FIG. 2. The voltage V(P) is averaged by an output filter, which comprises the choke coil L.sub.10 and a capacitor C.sub.10, into an output voltage V.sub.out free of alternating current components. The output voltage V.sub.out is indicated in relation to the drain-to-source voltage V.sub.ds (Q.sub.103) in FIG. 3. The drain-to-source voltage V.sub.ds (Q.sub.103) and the output voltage V.sub.out across the capacitor C.sub.10 are applied respectively to the terminals of the choke coil L.sub.10. The difference between the drain-to-source voltage V.sub.ds (Q.sub.103) and the output voltage V.sub.out determines a ripple current I(L.sub.10) flowing through the choke coil L.sub.10. The product of the ripple current I(L.sub.10) and the equivalent series resistance of the capacitor C.sub.10 approximately determines the value of an output ripple voltage.
FIG. 4 shows an output voltage of the conventional switching power supply shown in FIG. 2 with respect to a duty cycle thereof, as described above. As can be understood from FIG. 4, since the output voltage is proportional to the duty cycle, the duty cycle is set to nearly 0.5 generally when the input and output conditions are rated conditions. According to the waveform of the drain-to-source voltage V.sub.ds (Q.sub.103), the ratio of the period in which the drain-to-source voltage V.sub.ds (Q.sub.103) is nil, i.e., the sum of the off-times T.sub.off1, T.sub.off2, to the entire time, i.e., the operating period T.sub.1, is about 50%, so that the output ripple voltage is large. Stated otherwise, if a chock coil of a certain size is used to keep the output ripple voltage to a prescribed level, then the iron loss and copper loss of the chock coil are increased, indirectly lowering the efficiency of the switching power supply.
As described above, when a highly efficient switching power supply is to be constructed using the circuit shown in FIG. 2, the rectifying elements of the rectifying circuit connected to the secondary winding of the transformer cause a large power loss, and it is important to consider how the power loss can be reduced. Recently, efforts have been made to use synchronous rectifying transistors in place of the rectifying elements for there by reducing their conduction loss. It is also important to pay attention to effective energization of the synchronous rectifying transistors.